The highly effective three-way catalyst in widespread use in many modern vehicles with spark ignition engines simultaneously reduces nitrogen oxides (NO,) while also oxidizing carbon monoxide (CO) and hydrocarbons (HC). This catalyst function is enabled by tight control of exhaust air:fuel ratio (AFR).
Air:fuel equivalence ratio is determined by dividing the stoichiometric AFR by the actual AFR. Stoichiometry is defined as the chemically-correct AFR for complete theoretical combustion of all fuel with no remaining oxygen. As such, equivalence ratios greater than 1 always represent excess fuel conditions (rich) in the fuel-oxidizer mixture than stoichiometry, irrespective of the fuel and oxidizer being used. Equivalence ratios less than 1 represent an excess oxidizer condition (lean condition) in which complete theoretical combustion of all fuel leaves remaining oxygen in the combustion products. Net-lean combustion is defined as an exhaust condition in which the combination of rich and lean combustion results in common rich exhaust species (hydrogen, carbon monoxide, hydrocarbons) in the presence of more oxygen than what would be required for complete combustion (stoichiometric reaction). Enriched combustion occurs when the air:fuel equivalence ratio is raised via increasing the amount of fuel delivered to a cylinder by pre-existing injections or through the use of one of more additional injections.
Dithering the AFR slightly lean and slightly rich of stoichiometric operation allows the simultaneous control of NOx, CO, and HC in the conventional three-way catalyst. The excess oxygen present in lean diesel engine exhaust precludes the use of the three-way catalyst. The Lean NOx trap (LNT, also NOx adsorber or NOx Storage and Reduction (NSR) catalyst) is receiving considerable attention as a possible means to enable light- and heavy-duty diesel engines to meet future emissions standards. The LNT functions by storing NOx during normal lean operation (when excess oxygen in the exhaust hinders the chemical reduction of NOx). The LNT must be regenerated periodically by a rich excursion, a brief event in which the exhaust AFR is driven rich to achieve overall reducing conditions. The excess-fuel derived reductants (HCs, CO, hydrogen (H2)) cause the release and subsequent reduction of the stored NOx.
Several approaches to achieving this momentary rich excursion are being researched. In some cases raw fuel is sprayed into the exhaust (“in-pipe injection”) to enable regeneration. Synthesis gas has been shown to be a very effective reductant in both bench and full-scale laboratory experiments. As such, on-board fuel reformers are being researched as a potential means to provide CO and H2 for catalyst regeneration. In addition, in-cylinder injection of excess fuel, or rich combustion, is a common method, as well as the individual cylinder control method taught in this invention. All of these methods have unique advantages and disadvantages and are being considered as strategies for various diesel vehicle applications either individually or as coupled technologies.